Implement Document.scrollingElement API behind experimental feature flag

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 1248 matching lines @@
 
     HitTestResult result = hitTestInDocument(this, x, y);
     PositionWithAffinity positionWithAffinity = result.position();
     if (positionWithAffinity.position().isNull())
         return nullptr;
 
     Position rangeCompliantPosition = positionWithAffinity.position().parentAnchoredEquivalent();
     return Range::createAdjustedToTreeScope(*this, rangeCompliantPosition);
 }
 
+Element* Document::scrollingElement()
+{
+    if (RuntimeEnabledFeatures::scrollTopLeftInteropEnabled()) {
+        if (inQuirksMode()) {
+            updateLayoutIgnorePendingStylesheets();
+            if (body()->layoutBox() && body()->layoutBox()->hasOverflowClip())
+                return nullptr;
+
+            return body();
+        }
+
+        return documentElement();
+    }
+
+    return body();
+}
+
 /*
  * Performs three operations:
  *  1. Convert control characters to spaces
  *  2. Trim leading and trailing spaces
  *  3. Collapse internal whitespace.
  */
 template <typename CharacterType>
 static inline String canonicalizedTitle(Document* document, const String& title)
 {
     unsigned length = title.length();
@@ skipping 4422 matching lines @@
 #ifndef NDEBUG
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (Document* document : set)
         fprintf(stderr, "- Document %p URL: %s\n", document, document->url().string().utf8().data());
 }
 #endif